This invention relates to a current limiter for an electrical actuator such as an electric motor, and more particularly relates to a control for energizing the electrical actuator at a moment when an excessive current is supplied to the electrical actuator.
A chopping control is a very common method in the art so as to adjust the torque generated by the electrical actuator. The chopping control is obtained by changing a duty ratio, which is a ratio of an energizing period to a de-energizing period. When the chopping control is executed, the average level of the current through the electrical actuator is controlled.
The level of the current through the electrical actuator changes greatly according to load variation, temperature variation and supplied voltage variation, etc. Accordingly, an excessive current beyond a rating value might be supplied to the electrical actuator at times. If the excessive current is supplied to the electrical actuator for a long time, the electrical actuator might be damaged. Therefore, such excessive current should be limited.
Japanese Laid-open Patent Publication No. 60-98867 published on June 1, 1985 discloses a current limiter for limiting such excessive current. In this limiter, if the current through the electrical actuator exceeds the predetermined level, the duty ratio is decreased toward a small predetermined value.
However, after the current through the electrical actuator exceeds the predetermined level, pulsations may appear in the current as shown in FIG. 6 in the above described conventional limiter. Such phenomena is generated by a repeated procedure for limiting the excessive current. In other words, after the limiter decreases the current in accordance with the excessive current, if the current is under the predetermined level, the limiter stops operating. Then the current is increased gradually and exceeds the predetermined level so that the limiter decreases the current again.
If a large hysteresis is provided between one level for decreasing the current and another level for stopping limitation, it may be possible to prevent the pulsations from generating. However, the limited current is decreased to a very low level below the predetermined level so that the driving torque is reduced rapidly. If the electrical actuator is utilized for an electric power steering system, the pulsation of the current is represented as a vibration in the auxiliary torque whereby the power steering may feel unpleasant to a driver.
Further, the current through the electrical actuator may be changed in response to varied conditions such as supplied voltage, because the duty ratio for limiting the excessive current is a constant value. In an automobile, a battery is used for the power supply. Accordingly, the output voltage of the battery will be greatly changed. Therefore, if the duty ratio is decreased, an excessive current may be supplied if the output voltage is at a high level. Contrary, if the duty ratio for limiting the current is established at too small a value, an output torque can not be obtained when the output voltage is very low level.